Thanks for the answer!
If we don't want to have label displayed, we can just leave out
name field for the series since it is an optional field. In my opinion, it is better because it provides one more option for the users. It also feels more consistent in terms of API design.
In terms of visualization, it can be a workaround for my another issue regarding title (link) as I can use the label to show what line chart this is. I would love if I can minimize manual editing.
By the way, API doc does not mention this I think, so someone like me can get confused haha